Shaft induced electrical current is experienced in electric motors, and commonly in three-phase motors driven by variable speed drives. Variable speed drives utilize pulse width modulation technology to vary the speed of AC motors, thereby allowing use of less-expensive AC motors in applications where more expensive DC motors had been used previously. A drawback to the use of AC motors with variable speed drives is that higher common mode voltage (CMV) is generated by the variable speed drive, which increases shaft induced currents.
Voltage on the motor shaft induces current flow through the shaft bearings to the motor frame and then to ground. While the motor is running, the bearings become more resistive to current flow, causing a buildup of charge on the shaft surfaces. Over a short period of time, the CMV causes electrical charges to build to a high level. As the electrical charges pass the threshold level of the least electrically resistant path, sometimes through the ball bearings on the shaft, an instantaneous burst or discharge of electrical energy passes through the least resistant path. This discharge causes electric discharge machining (EDM), which can damage the surfaces of the bearing races and the balls in the bearing if the least resistant path is through the bearings. The electrical energy burst creates fusion craters, and particulate from the crater formation remains inside the sealed bearing. Both the fusion crater and the particulate material in the bearing act to disturb the free flow rotation of the bearing, which can lead to physical damage and premature bearing failure.
A number of mitigation technologies have been used in attempts to overcome this problem. Known attempts include using conductive bearing grease, insulating the bearings and using copper/phosphorus brushes and a Faraday shield. A common, somewhat cost-effective solution is to ground the shaft using spring-loaded copper brushes that provide a continuous flow of current to ground. Copper brushes can wear out rapidly, requiring frequent, periodic service and replacement. Additionally, oxide build-up on the shaft and other barriers between the brushes and the shaft reduce the current flow and cause a burst of electrical energy across the brush and shaft. Spring-loaded brushes also tend to vibrate due to alternating frictional relationships between the brush and the shaft surface. Vibration of the brushes, from whatever cause, can result in undesirable sparking.
The aforementioned related applications disclose grounding brushes that include conductive filaments in a holder surrounding the shaft. The brush can be used as a non-contacting ionizer to reduce the amount of electrical charges on the isolated shaft or on an isolated roller.
Still other types of grounding brush assemblies are known. When supplied as a continuous ring encircling a shaft, the grounding brush assembly can accumulate oil or other liquids around shaft. The accumulated liquid can interfere with the operation and performance of the grounding brush. The contaminants can migrate to bearings on the shaft and can cause damage or deteriorated performance. Removal of the assembly for periodic cleaning is time-consuming and may not be practical for substantially continuously operating equipment.
What is needed in the art is a grounding system that can be used effectively for a prolonged period of time, without accumulating oil or other liquids adjacent the shaft on which it is installed.